The present invention relates to semiconductor devices and to a manufacturing method of semiconductor devices that makes use of semiconductor compounds of Group III-V elements, such as GaAs, and in particular, to an improvement in the process of forming a p- or n-type ion implanted layer.
Because of their large electron mobility, semiconductor compounds of Group III-V elements such as GaAs have been investigated for their application to electronic devices that are operated at high speed.
In such devices, there is required an especially high degree of controllability so that the ion implantation method is now being adopted almost exclusively in place of Zn diffusion method that has been used widely in the past.
For instance, in the manufacture of HBT (Hetero-junction Bipolar Transistor), ion implantation of p-type impurities is performed in the process of formation of a high concentration external base layer for taking out a base electrode. In this case, as a kind of ion for the p-type impurities, use is made of beryllium (Be) or magnesium (Mg) which possess a relatively large projection range.
However, one technical problem associated with this existing technique is that it is difficult to form a high concentration p-type layer by means of the ion implantation method.
FIG. 1 illustrates the relationship between the sheet carrier concentration and the amount of ions implanted for a p-type layer which is obtained by implanting Mg in a semi-insulating GaAs substrate with an accelerating energy of 180 KeV, and by applying a heat treatment by irradiation with a halogen lamp, while suppressing dissociation of As by covering the surface with an SiO.sub.2 film obtained by CVD (Chemical Vapor Deposition) method. Starting with the implanted amount that exceeds 2.times.10.sup.14 atoms/cm.sup.2, the proportionality between the sheet carrier concentration and the implanted amount begins to deteriorate, and the sheet carrier concentration reaches only the low level of 3.times.10.sup.14 /cm.sup.2 at the implanted amount of 2.times.10.sup.15 atoms/cm.sup.2. This means that only 15% of the implanted Mg is acting as acceptors.
FIG. 2 shows the distribution of the carrier concentration (solid line) for the implantation of 2.times.10.sup.15 atoms/cm.sup.2. According to the curve determined by the LSS theory, which is represented by a broken line, the peak carrier concentration should reach 1.times.10.sup.20 /cm.sup.3. The measured carrier concentration, however, remains at 5.times.10.sup.18 /cm.sup.3. This figure also shows an enormous diffusion that occurred during the heat treatment. This diffusion also means that the amount of activated Mg as acceptors is relatively small in quantity.
As for the cause of the low rate of activation of Mg, it is necessary to consider the deviation from the stoichiometric composition of GaAs. In order for Mg to act as an acceptor in GaAs, it is necessary to infiltrate into the Ga lattice site and combine with As. In a GaAs crystal, the density of Ga atom is 2.2.times.10.sup.22 /cm.sup.3 so that when Mg is implanted to a concentration of about 1.times.10.sup.20 /cm.sup.3 as shown in FIG. 2, it becomes necessary for all of the Mg atoms to activate as acceptors, either by the shift of about 1% of Ga atoms to interlattice positions or by the diffusion into the SiO.sub.2 film formed on the surface. However, these changes do not occur to a sufficient extent under usual conditions.
There are some problems in the implanting of Be or Silicon (Si). Though Si infiltrates into the Ga site in the same way as Mg or Be, it behaves as a donor in the GaAs material.
There have also been investigations concerning the implanting of As, which is a compositional element of the GaAs substrate, together with Zn or Si to improve the carrier concentration and/or diffusion. This method, however, is not preferable because of the damage caused by As ion implantation. As has the atomic number 33, which is large compared with 4 of Be, 12 of Mg or 14 of Si. Therefore, the energy of implanting As becomes enormous if attempts are made to obtain a project range comparable with that for Be, Mg or Si. The carrier concentration is limited due to the damage. Therefore, ion implantation using As is not suitable.